# Compute the surface charge density on the belt

I need the answer to this question

A charge belt, 50cm wide, travels at 40m/s between a source of charge and a charging object at a rate corresponding to 100microA . Compute the surface charge density on the belt

Some one told me the answer may be 5 microC/m2 but how ??

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I try this but i dont no if it is right
J=I/A=100*10^-9 /0.5*1
also J=(ne) V
then ne(q)=J/v
and then i take the q as the surface charge density
and it well = 5*10^-9

The units do not come out right at all in that last equation.

You know the current and that is just dq/dt. That is the same as saying dq charge passes by in a time dt. So draw a picture of how much charge passes by in a time dt, knowing the width of the belt and how fast it is moving.

Thanx Nickjer For your help I finally get it